Vacancies: -vacant atomic sites in a structure. Self-Interstitials: -"extra" atoms positioned between atomic sites. POINT DEFECTS CHAPTER 4: IMPERFECTIONS IN SOLIDS

Two outcomes if impurity (B) added to host (A): Solid solution of B in A (i.e., random dist. of point defects) Solid solution of B in A plus particles of a new phase (usually for a larger amount of B) OR Substitutional alloy (e.g., Cu in Ni) Interstitial alloy (e.g., C in Fe) Second phase particle --different composition --often different structure. POINT DEFECTS IN ALLOYS

Low energy electron microscope view of a (111) surface of Cu. Sn islands move along the surface and "alloy" the Cu with Sn atoms, to make "bronze". The islands continually move into "unalloyed" regions and leave tiny bronze particles in their wake. Eventually, the islands disappear. ALLOYING A SURFACE

Dislocations slip planes incrementally... The dislocation line (the moving red dot)......separates slipped material on the left from unslipped material on the right. INCREMENTAL SLIP LINE DEFECTS Atomic view of edge dislocation motion from left to right as a crystal is sheared.

DISLOCATIONS EDGE DISLOCATIONS

SCREW DISLOCATIONS

Grain boundaries: are boundaries between crystals. are produced by the solidification process, for example. have a change in crystal orientation across them. impede dislocation motion. Schematic ~ 8cm Metal Ingot AREA DEFECTS: GRAIN BOUNDARIES

Grain boundaries... are imperfections, are more susceptible to etching, may be revealed as dark lines, change direction in a polycrystal. OPTICAL MICROSCOPY

RESOLUTION RANGES